refactor: Rename click property to on_click (closes #139)

Breaking change: callback property standardized to on_* pattern.
- `drawable.click` → `drawable.on_click`

Updated all C++ bindings (8 files) and Python test usages.
Note: src/scripts changes tracked separately (in .gitignore).

This establishes the naming pattern for future callbacks:
on_click, on_enter, on_exit, on_move, on_key, etc.

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Co-Authored-By: Claude <noreply@anthropic.com>

src/UIGrid.cpp

@@ -1478,8 +1478,8 @@ PyGetSetDef UIGrid::getsetters[] = {
     {"center_y", (getter)UIGrid::get_float_member, (setter)UIGrid::set_float_member, "center of the view Y-coordinate", (void*)5},
     {"zoom", (getter)UIGrid::get_float_member, (setter)UIGrid::set_float_member, "zoom factor for displaying the Grid", (void*)6},
 
-    {"click", (getter)UIDrawable::get_click, (setter)UIDrawable::set_click,
-     MCRF_PROPERTY(click,
+    {"on_click", (getter)UIDrawable::get_click, (setter)UIDrawable::set_click,
+     MCRF_PROPERTY(on_click,
          "Callable executed when object is clicked. "
          "Function receives (x, y) coordinates of click."
      ), (void*)PyObjectsEnum::UIGRID},